Microwave broadband switching assembly



March 24, 1970 s co cc o ET AL 3,503,015

MICROWAVE BROADBAND SWITCHING ASSEMBLY Filed May 5, 1969 lo 3 3140 32B14 A y I Q o 0 A fil FIGZ 36A 36B 36C 360 I0 W 32A 25 G 1? 41 I 9orogiwwm/wsm ATTO RNEYS United States Patent 3,503,015 MICROWAVEBROADBAND SWITCHING ASSEMBLY Salvatore G. Coraccio, Carlisle, Philip E.King, Acton,

and Peter A. Rizzi, Dedham, Mass., assignors to Alpha Industries, Inc,Newton Upper Falls, Mass., 21 corporation of Delaware Filed May 5, 1969,Ser. No. 821,651 Int. Cl. H01p 5/12 US. Cl. 3337 12 Claims ABSTRACT OFTHE DISCLOSURE A broadband microwave switch includes a pair of coaxialterminals, an outer conductor defining a cavity and an inner conductorextending through the cavity and capacitively coupling the coaxialterminals to one another. Within the cavity, a plurality of diode chipshave one terminal connected to this inner conductor. At least one ofthese diode chips is capactively insulated at the other terminal fromthe outer conductor. The remaining diode chips have their otherterminals respectively connected to the outer conductor. A conductingwire extends through and is insulated from the outer conductor and intothe cavity connecting with the terminal of the diode chip which iscapacitively insulated from the outer conductor to allow a bias signalto be supplied to the diode chips. Portions of the inner conductorwithin the cavity coact with the outer conductor to define substantiallyinductive transmission line segments. The substantially inductivesegments coact with the effective capacitance of the nonconducting diodechips to define a substantially reflectionless filter circuit.

Background of the invention The present invention relates in general tomicrowave switches and more particularly concerns a novel broadbandmicrowave switch of high electrical performance, small physical formwhich is easy and inexpensive to fabricate in large and small quantitieswhile maintaining a high degree of repeatability in electricalperformance.

One well known type of diode switch comprises an inner conductor withina cavity which is capacitively coupled to the coaxial terminals andhaving cartridge or encased type diodes connected thereto. Cartridgecrystals require a crystal holder and elaborate spring mechanisms tocreate proper contact between the crystal and the inner conductor.Moreover these large crystals need elaborate biasing methods introducingfurther fabrication problems and usually unwanted reflections atmicrowave frequencies. While these techniques have not foreclosedsatisfactory diode switch operation, they do introduce mechanical andelectrical problems which are disadvantageous. The mechanical structuresheretofore known are not only complex, but also present considerableproblems in connection with R-F impedance matching, particularly whenbroad bandwidths are desired.

These problems are caused mainly by the relatively large size of thecrystals and their inherent mismatch with respect to the standardimpedances found in the microwave field (i.e. 50 ohms).

It is an important object of the invention to overcome limitations ofthe prior art and provide a broadband diode switch of relatively smallsize that is adapted for utilization of standard miniature diode chips.It is another object to provide a relatively simple method of lowrelection bias insertion.

A further object of this invention is to provide a broadband diodeswitch of relatively low SWR which is susceptible of sealed or unsealedoperation.

A further object of this invention is to provide a broadband diodeswitch of relatively low insertion loss and yet achieving high isolationwhen bias is applied.

Another object of this invention is to provide a broadband diode switchadaptable to use of a variety of diode chips.

A further object of this invention is to provide a broadband diodeswitch incorporating bias carrying means within the circuit.

Another object of the invention is to achieve one or more of thepreceding objects while keeping costs relatively low.

Summary of the invention According to the invention, two coaxial outerterminals are connected to an outer conductor defining a cavity. Thesignal conductors inside each of the coaxial outer terminals arecapacitively coupled to an inner conductor extending through the cavity.A plurality of diode chips are inside the cavity. One of these diodechips has one terminal connected to the inner conductor extendingthrough the cavity and the other terminal connected to a capacitorinsulating the diode chip from the outer conductor. The remaining diodechips have one terminal connected to the inner conductor extendingthrough the cavity and the remaining terminals respectively connected tothe outer conductor defining the cavity. Bias is supplied by means of aconducting wire extending through and insulated from the wall of thecavity defining outer conductor. This wire extends into the cavity andis connected to the diode chip which is capacitively insulated from theouter conductor at the terminal which is connected to the capacitor.Portions of the inner conductor within the cavity coact with the outerconductor to define substantially inductive transmission line segments.The substantially inductive segments coact with the effectivecapacitance of the nonconducting diode chips to define a substantiallyreflectionless filter circuit.

In a modification of the present invention, there are only two diodechips, one being capacitively insulated from the outer conductor; theother having one terminal directly connected to the outer conductor. Theremaining terminals of both diode chips are connected to the innerconductor extending through the cavity which is capacitively coupled tothe signal conductors in each of the coaxial outer terminals. Bias issupplied in the same manner as above to the diode chip beingcapacitively insulated from the outer conductor at the terminalconnected to the capacitor.

These and other objects and advantages of the present invention will bemore clearly understood when considered in conjunction with theaccompanying drawings in which:

Brief description of the drawings FIG. 1 is a longitudinal sectionalview of the broadband diode switch having two coaxial terminalsaccording to the invention;

FIG. 2 is a sectional view through section line 2-2 of FIG. 1;

FIG. 3 is a schematic circuit diagram of the embodiment of FIGS. 1 and2; and

FIG. 4 is a schematic circuit diagram of a modification of theinvention.

Detailed description of the preferred embodiments Correspondingreference symbols will be used throughout the drawings to indicatecorresponding elements where applicable.

With reference now to the drawings and more particularly to FIG. 1thereof, there is shown a longitudinal sectional view of the embodimentof the invention in which the two coaxial terminals A and B of themicrowave switch are diagrammatically represented. Signal conductors 11and 12 of coaxial terminals A and B, respectively, are capacitivelycoupled to inner conductor extending through the cavity at points 13 and14, respectively. The capacitance at points 13 and 14 between innerconductor 10 and signal conductors 11 and 12 presents a low impedance atthe microwave frequencies of interest, and a high impedance at thebiasing signal frequency.

Casing 23 defines the outer conductor for inner conductor 10 and alsointercouples the reference terminals 21 and 22 of coaxial terminals Aand B respectively. Capacitive insulator 40 is attached by one terminalto the floor of the cavity formed by outer conductor casing 23. Diodechip 31 is placed upon and connected to the remaining terminal ofcapacitor insulator 40 and has its remaining terminal connected to innerconductor 10 extending through the cavity. Dode chips 32A and 32B areconnected to the floor of the cavity formed by outer conductor casing 23and have their remaining terminals respectively connected to the innerconductor 10 extending through the cavity. Conductor is a thinconducting wire extending through and insulated from the wall of outerconductor casing 23 into the cavity and connects to the terminal ofdiode chip 31 which is attached to the :apacitive insulator 40.

FIG. 2 is a sectional view through the embodiment of FIG. 1 betterillustrating the relationship among capacitive insulator 40, the floorof the cavity formed by outer :onductor casing 23, diode chip 31, anddiode chips 32A and 32B, and inner conductor 10 extending through the:avity. This view shows how conducting wire 15 attaches to the junctionof capacitive insulator 40 and diode chip 31 at point 17. Conductor 16is attached to casing 23 so that when a potential difference occursbetween conductors 15 and 16, a bias signal is developed across thediode chips.

FIG. 3 is a schematic circuit diagram of the embodiment of FIGS. 1 and 2illustrating the diode chips in their nonconducting states assubstantially capacitive elements and further showing how diode chips 31and 32A are in series combination with respect to a bias signal suppliedacross conductors 15 and 16, as are diode chips 31 and 32B. The diodechips may be arranged so that the polarization of the diode chips allowseach series comhination to conduct when a positive bias signal issupplied to conductors 15. When the diode chips are in their conductingmode, as caused by the positive bias signal being applied to conductor15, diode chips 32A and 32B are virtual short circuits to the R-Ffrequencies, thereby causing virtually no signal to pass through thedevice.

Conductor 10 may be considered as a plurality of serially-connectedsubstantially inductive distributed parameter transmission lineelements, represented by inductors 36A, 36B, 36C and 36D in FIG. 3. Thevalues of the respective inductors are chosen so that, upon coactingwith the shunt-connected substantially capacitive non- :onducting diodechips 31, 32A and 32B, a substantially refiectionless low pass filtercircuit is created.

FIG. 4 is a schematic circuit diagram of a modification of the inventionin which only two diode chips, 31 and 32A, are used. The diode chips arearranged, as in FIG. 3, in series with respect to a bias signal appliedto conductors 15 and 16, but they may be reversed in polarity therebyrequiring a negative bias signal to be applied to conductor 15 if thediode chips are to conduct.

Again the diode chips in their nonconducting states are represented assubstantially capacitive elements. The capacitors coact with thesubstantially inductive segments of the inner conductor 46A, 46B and 46Cforming a substantially refiectionless low pass filter circuit.

In a specific embodiment of the invention 50 ohm type axial terminalswere used with an outer conductor casing of Kovar material, 0.557 inchlong. The cavity formed within the outer conductor was 0.337 by 0.150inch. 200 pt. c pacit s we e us d a c p g c p c t s c n cting the innerconductor to the signal conductors of the connectors. 0.005 inch by0.001 inch ribbon was used as the inner conductor within the cavity. A200 pf. capacitor was used to insulate one diode chip from the outerconductor. Three Si PIN diode chips were used within the cavity. Thediode chips required a bias current of 50 ma. positive in order toswitch the diodes to their conducting states. The diode switch operatedover a frequency range of 0.5 to 12.4 gHz. with an insertion loss of 2.0db and 2.0 maximum SWR. When the diode chips were switched to theirconducting states an isolation of 70 db was achieved.

An important feature of the invention is the adaptability of thestructure to accommodating terminals at different locations. FIG. 1shows terminals being brought out at opposite points of the longitudinalaxis of the diode switch. But the invention operates equally well withthe terminals in space quadrature or at other suitable angles.

Another important feature of the invention is the adaptability of thediode switch to accommodating various types of diode chips. Varactors,PIN diodes or any other unilaterally conducting device may be useddepending upon the particular design criteria (modulation levels,switching speed, power handling capability or other constraints).Different types of diode chips may be intermingled within the switchitself. The proper values for the substantially inductive conductors maybe calculated knowing the capacitance of the nonconducting diode chips,thus creating a substantially refiectionless filter CII'CUII.

Another important feature of the invention is the freedom of thefrequency response of the switch from perturbations caused by the biasinsertion structure. The bias is applied within the circuit at a pointessentially maintained at R-F ground potential, effectively isolatingthe bias carrying circuits from the R-F transmission portion of thecircuit.

The invention is illustrated with a rectangular cavity formed by theouter casing. The cavity can be cylindrical or in any other suitableshape. The diode switch may also be constructed of two parallel platesforming a transmission line with the diode chips and insulatingcapacitor being mounted on one or both of the plates. The diode switchmay be constructed in microstrip or any other TEM waveguideconfiguration.

The diode chips may be mounted within the cavity with such polarity asto conduct when either positive or negative D-C bias is applied.

The terminals of the invention may be coaxial, as illustrated above, lugterminals, or any type of terminals to allow convenient use of theinvention.

Other modifications and uses of and departures from the specificembodiments described herein may be practiced by those skilled in theart without departing from the inventive concepts. Consequently, theinvention is to be construed as limited solely by the spirit and scopeof the appended claims.

What is claimed is:

1. A broadband microwave switch comprising:

means defining first and second terminal pairs,

first conducting means defining a cavity and normally maintained at areference potential intercoupling reference ones of each of saidterminal pairs,

each of said terminal pairs having a signal terminal,

second conducting means within said cavity for intercoupling said signalterminals,

means defining a plurality of diode chips,

a first of said diode chips having a first terminal connected to saidsecond conducting means within said cavity and a second terminalconnected to means insulating said diode chip from said first conductingmeans and coacting therewith to define capacitive means,

the remainder of said diode chips having one terminal connected to saidsecond conducting means within said cavity and the other terminalconnected to said first conducting means,

and means defining a conducting lead extending through said firstconducting means defining said cavity and insulatedly separatedtherefrom and into said cavity connecting to said first diode chip atsaid second terminal for carrying a biasing signal.

2. A broadband microwave switch in accordance with claim 1 having threeof said diode chips wherein said first diode chip is interposed betweenthe remaining two diode chips.

3. A broadband microwave switch in accordance with claim 1 having two ofsaid diode chips.

4. A broadband microwave switch according to claim 1 wherein said secondconducting means includes inductive means coacting with the effectivecapacitance of said diode chips in their nonconducting states to definea substantially reflectionless filter circuit.

5. A broadband microwave switch according to claim 1 wherein saidcapacitive means comprises means for maintaining said second terminal ofsaid first diode chip essentially at R-F ground potential.

6. A broadband microwave switch according to claim 1 and furthercomprising capacitive means intercoupling said second conducting meanswith said signal terminals.

7. A broadband microwave switch according to claim 5 wherein said secondconducting means includes inductive means coacting with the effectivecapacitance of said diode chips in their nonconducting states to definea substantially refiectionless filter circuit.

8. A broadband microwave switch according to claim 7 and furthercomprising capacitive means intercoupling said second conducting meanswith said signal terminals.

9. A broadband microwave switch according to claim 8 comprising three ofsaid diode chips wherein said first diode chip is interposed between theremainder of said diode chips.

10. A broadband microwave switch according to claim 8 comprising two ofsaid diode chips.

11. A broadband microwave switch according to claim 9 wherein saidinductive means includes means serially interposed between said diodechips.

12. A broadband microwave switch according to claim 10 wherein saidinductive means includes means serially interposed between said diodechips.

References Cited UNITED STATES PATENTS 3,337,820 8/1967 Harper 307-32OX3,417,351 12/1968 Piazza 33373 ELI LIEBERMAN, Primary Examiner MARVINNUSSBAUM, Assistant Examiner U.S. Cl. X.R

